1. Technical Field
The present invention relates to a knee-joint prosthesis for permanent anchoring in the bone tissue of a knee-joint in the human body, on the one hand consisting of a femur part which comprises an articulation element for the femur with a convex joint surface, an elongate intermediate element with a first side intended to bear against a surface of the bone tissue, and a second, opposite side intended to bear against an inner surface of the articulation element, in which respect the articulation element is arranged securely on the intermediate element by means of a positive lock, and at least one anchoring element intended to be introduced into a recessed channel in the bone tissue and to be connected to this for anchoring the intermediate element and, thus, also the articulation element in the femur, and, on the other hand, a tibia part with a concave joint surface for cooperation with the convex joint surface of the femur part, and at least one anchoring element intended to be introduced into a recessed channel in the bone tissue for anchoring the tibia part in the tibia.
2. Technical Problem
On the market and in clinical use there are at present a large number of knee-joint prosthesis intended for permanent anchoring in the bone tissue. These prosthesis are intended as replacement material in cases of morbid defects which have developed as a result of joint-destructive disorders, either degenerative disorders or rheumatism. The joint replacement material, which can replace parts of or all of the knee-joint, generally consists of a metal and polyethylene component, but there are also other material combinations such as, for example, a ceramic articulating with polyethylene. The metal component in artificial joints consists almost exclusively of one of three alloy types, so-called super alloys. These three alloys are stainless steel, which is now generally nitrogen-based with a high chromium content, cobalt chromium alloys or the titanium alloy Ti 6AI 4V. All these alloys contain metals which are potentially toxic. For example, aluminium has been shown to be neurotoxic. Chromium, which is included in steel and cobalt-based alloys, is considered to be carcinogenic. Nickel, which is also included in the same two alloys, is a powerful allergen. In spite of the potential risks and despite the fact that certain of these alloys have been used for as long a period as 30 years or more, there have been surprisingly few reports which have given convincing evidence of material complications. However, in this context it should be added that it is only in the last 20 years that joint replacements of the said material have been available on a large scale.
In general it may be said that the knee-joint replacements available on the market today give reasonable results over a ten-year period, but the experiences with this type of material are largely limited to older patients with relatively low levels of activity. Complications have been reported to a fairly large extent. It is scarcely by chance that there is an abundance of different knee-prosthesis designs of different materials and with different biomechanical solutions. As regards previously known knee-joint replacements, none can be said to represent a definitive solution with respect to anchoring in the bone tissue, wear-resistance, joint stabilization or material strength. The central problem would appear to be the anchoring stability of the prosthesis components.
It is nowadays agreed that an as exact as possible contact fitting should be created between prosthesis component and bone tissue, regardless of whether the contact is established via a filling material or not. Filling material in the form of polymethyl methacrylate (PMMA) permits a more or less exact initial contact fitting between foreign material and bone tissue in the wider sense. Bone cement (PMMA) in the low-viscous form can, under pressure, be made to penetrate into the bone tissue and create a positive lock on the microplane. Since PMMA has a good adhesion to the prosthesis components, in this way a very good initial fixation is obtained. However, the disadvantage with PMMA is that chemical and thermal action on the bone tissue causes tissue damage and bone loss, in which respect the exact contact fitting initially obtained is completely or partially lost. Moreover, the bone cement has stability properties which, in the long term, lead to obvious risks of brittleness fractures with, as a consequence, inadequate contact surface with the bone tissue.
On account of observed and suspected complications secondary to the use of bone cement as filling material, for a good ten years laboratory work has been carried out on a number of prosthesis types which are anchored cement-free. The idea was to create a positive lock by using a more or less exact operative technique to adapt the contours of the bone ends, so that a good fit is obtained between the prosthesis components and the bone end. The positive lock which has been sought between non-cemented prosthesis and bone has been present on the microplane and macroplane in different prosthesis designs. An example of a positive lock on the macroplane is the ICLH prosthesis and its successors, where an operative technique rather than the prosthesis design itself afforded the conditions for a positive lock. The operative technique offered possibilities of guiding the cutting instruments in such a way that, on the one hand, it was possible to position the prosthesis exactly in relation to the mechanical axis of the knee-joint and, on the other hand, a bone-end contour was obtained which matched the smooth inner contour of the prosthesis. In a number of other prosthesis systems this principle has been used for macro-locking, but, in addition, a positive locking on the microplane has been sought by means of that surface of the prosthesis facing the bone tissue having a porous structure with opportunities for tissue to grow in. The in-growing tissue which appears is considered by prosthesis designers and manufacturers to be different from bone tissue, by reason of which a three-dimensional positive lock is obtained on the microplane with ideal stabilization, including osseointegration. However, this hope has not been fulfilled, judging from the histological findings of various retrieval studies. The in-growing of bone has, thus, only been observed in a limited sense. It has been found that femur prosthesis often have bone growing into the pore system, while the tibia prosthesis, which is the prosthesis component most prone to loosening, relatively rarely has such bone ingrowth. Furthermore, the bone ingrowth which does take place does not create direct contact with the bone tissue other than at points, and is separated from the tissue by a thin layer of fibrous tissue.
The fact that both cemented knee prosthesis and prosthesis intended for cement-free anchoring do not establish direct contact with the bone bed has been very clearly demonstrated by Ryd and co-workers from Lund. Using a stereoscopic x-ray method it was possible to show that both cemented and non-cemented prosthesis migrate, i.e. "slacken", especially during the first year after the implantation. It was also possible to demonstrate an instability of the prosthesis component. Thus, dynamically loaded knee-joint prosthesis move in relation to the bed of the bone. The clinical implications of Ryd's work are uncertain at present, but it is clear that both cemented and non-cemented prosthesis anchor in connective tissue, i.e. have a layer of connective tissue between the prosthesis component and bone seat.
The aim of the invention for which a patent is applied for is to improve the anchoring situation of knee-joint prosthesis by providing for an exact match between prosthesis and bone tissue, i.e. a direct contact between metal and bone without any intermediate layer of connective tissue. This type of anchoring, which is called osseointegration, has been shown to be practicable in denture reconstruction using threaded titanium implants and is described by Albrektsson, T., Br.ang.nemark, P.-I., Hansson, H-A. and Lindstrom, J. 1981, "Osseointegrated titanium implants. Requirements for ensuring a longlasting, direct bone anchorage in man," Acta Orthop Scand 52,155-170. Br.ang.nemark and co-workers have thus been able to show that, by adhering to certain basic principles, it is possible to achieve osseointegration and also to maintain this anchorage for a long period of time in the human jaw. In this connection clinical experience goes back over 20 years. The corner-stones which were considered by Br.ang.nemark and co-workers to be the conditions for achieving and maintaining osseointegration in cases of skeletal implantation are the use of prosthesis components made of commercially pure titanium, the screw design of the anchoring elements, and the fact that these and functional elements are introduced in two separate operations with an interval between them of about three months. The idea is that the incorporation of the implant is promoted by the fact that stress is not transmitted over the interface zone between bone and metal during the incorporation phase. An additional corner-stone in the reconstruction system is that the implant is introduced with the least possible tissue trauma, which presupposes a specially formulated operative technique in which the tissue is not subjected to supraliminal thermal and mechanical stimuli and where, moreover, the tissue is protected from tissue ischaemia by taking account of the vascular soft parts.
In an extremely wide-ranging experimental study Albrektsson and co-workers analysed the conditions for obtaining osseointegration in respect of the knee-joint. In this experimental work they assessed the effect of a number of different variables on incorporation parameters related to the achievement of osseointegration. They were able to establish that the use of commercially pure titanium probably creates better conditions for osseointegration than do alternative implant metals. They were also able to establish that the matching of the surgical defect to the implant is essential. The preconditions for osseointegration were also shown to be considerably more favourable if screw connections were used instead of alternative connections such as spikes or perforated plates. They also succeeded in showing, and this is an entirely new observation, that joint replacements made of pure titanium can be made to osseointegrate in a onestage process. This is, of course, important information, since the two-stage process, which was considered by Br.ang.nemark and co-workers as a precondition for osseointegration, entails practical difficulties, especially in a joint application. It is thus only with great . reluctance that surgery will be carried out on a knee-joint in two separate operations, bearing in mind the risk of interference with sliding surfaces and displacement layers in the soft parts. The risk of development of capsule fibrosis, with reduction in the volume of the joint cavity, increases with each intra-articular intervention.
Therefore, in summary, it may be said that the aim of the present invention was to develop an osseointegrated knee-joint prosthesis which satisfies some, but not all, of the requirements which have been regarded as being preconditions for osseointegration of jaw implants. It is desired to avoid a two-stage process in the installation of the artificial joint replacement material, at least at the femur end, where a two-stage process is difficult to carry out without the joint being opened twice, and where, moreover, experimental studies indicate that simultaneous installation of anchoring and functional elements is consistent with osseointegrated anchorage. To the extent that two separate operations have been thought necessary for achieving osseointegration at the tibia end, a technique has been sought for carrying out the first intervention extra-articularly, i.e. without going into the knee-joint. The objective has been that the surgical work required to instal the knee-joint prosthesis, in respect of which a patent is been applied for, on the one hand should comprise a first operation in which anchoring screws are introduced extra-articularly into the tibia end and, on the other hand, a second operation about three months later, when both anchoring element and joint element are installed in the femur end and the joint element is connected to the anchoring element osseointegrated at the tibia end.
The principle in respect of which a patent is applied for differs from other non-cemented prosthesis designs for the knee-joint. The principle is based partly on previously known principles, see for example SE 450,336 or EP 0,183,669, but, as emerges from the text below, we have not only improved but also radically altered the known design solution which has several important disadvantages. The most important objection is that no been given regarding methods for cutting bone tissue in a way which permits an exact initial contact fitting by atraumatic techniques; nor has it been possible to offer any solution to the positioning problem which is fundamental to achieving a preserved, osseointegrated anchorage permitting biomechanics. Without suitable cutting techniques and positioning techniques, it may be said that the design previously patented, or for which a patent is applied for, is no more than a drawing-board product. Moreover, in the cited design solutions, insufficient consideration has been given to wearing problems connected with the use of commercially pure titanium in sliding surfaces against polyethylene. It is true that it has been considered possible to compensate for the wear resistance of pure titanium by means of surface modification of the titanium joint surface In extensive laboratory tests carried out during 1987 it was found that surface modification of pure titanium, which was previously considered possible for achieving acceptable wear resistance, does not provide such resistance. Instead, all experimental experience indicates that that part of the artificial joint which bears against plastic or other material cannot consist of commercially pure titanium, which is altogether too soft, regardless of whether or not its surface has been hardened. The tests carried out show that one precondition for achieving sufficient wear resistance is that the articular head of the prosthesis be made of surface-modified titanium alloy. In theory, from the point of view of wear, it should also be possible for the articular head to be made of cobalt-based alloy, but then there is also an obvious risk of galvanic corrosion.
Moreover, it may be added that these previously known knee-prosthesis designs involve, in addition to the abovementioned disadvantages, a number of inadequately thought-out solutions, particularly as regards mechanical connections entailing risks of insufficient prosthesis function, regardless of whether the implant has or has not been osseointegrated. As far as the patented design solution is concerned, it is also doubtful whether conditions for osseointegration exist at all, bearing in mind that the anchoring elements are not made up of screws of the type with which one has experience in jaw reconstruction material, but rather of grooved plugs in the form of a double barrel where the plugs are joined with a plate. This type of implant design has been shown by Carlsson and co-wcrkers (1986) to give poor conditions for osseointegration, and it may thus be said that the whole design solution which is offered in this known design solution can be called into question It is at least clear that, regardless of whether a two-stage process is used or not, there is no experimental basis for assuming that osseointegration can be achieved with this design other than in exceptional cases
As regards the prosthesis design according to EP 0,183,669 it may be stated that this too has a number of disadvantages, even if not quite as obvious as in the version according to SE 450,336. The prosthesis design, as described in SE 450,336, comprises several inadequate mechanical connections. Moreover, the recommended reconstruction procedure creates an unfavourable biomechanical situation with a necessary moment arm which threatens to thwart the osseointegration which was probably obtained initially. In the version according to EP 0,183,669 an attempt has been made to compensate for this by building in a superficial, cross-wise, flat connection arm between the two grooved anchoring elements in each tibia condyle. This certainly relieves some of the stress which was obtained by the moment arm which is formed between the anchoring element and the periphery of the plateau of the tibia. If there is an excess load medially along the tibia prosthesis, then the transverse arm reduces the torsional moment against the medial anchoring element, but especially at the front end where the arm is positioned, and at the same time a cranially-directed force vector is obtained on the contralateral anchoring element, which risks dislodging the osseointegration on the side. Thus, the stress distribution in the proximal tibia is by no means uniform, nor can one reliably avoid excess loads which have disastrous consequences for the osseointegration connection. In addition, both the connection arm and the anterior-posterior direction of the anchoring elements mean that it is not possible to prevent outwardly projecting implant material from lying in close contact with the skin, with the risk of perforation upon direct force, and then also the risk of contamination and secondary infection.
As regards the mechanical connections between anchoring element and joint element, as described in EP 0,183,669, it may be said that there are several unreliable points, in particular concerning the connection between the articulation part of the femur prosthesis and the intermediate element which, on the one hand, causes production-technology difficulties with guide spikes and spring-action pins and, on the other hand, provides insufficient positive locking, with a risk of the prosthesis tipping in the lateral direction unless it is certain that the bone walls in the bone bed of the articulation element afford sufficient positive locking.
The intermediate element, which is L-shaped in the previous version and therefore involves difficulties both in production technology and in surgical techniques, has therefore no force-transmission lock of the type which is included in the prosthesis design solution according to the present invention. In the previous version a spring-action pin was relied upon to sufficiently compensate for shear stresses in the anterior-posterior direction. It is doubtful whether a spring pin can offer the anchoring stability which is required between articulation element and intermediate element, and it is also doubtful whether the spring principle functions at all. If, in conjunction with the pressing-on of the articulation element, tissue fragments enter the hole for the spring pin, there is a risk that the pin will not go into its hole at all, or will only go into this partially, with inadequate anchoring stability as a result. Another disadvantage of a spring-action pin of the type mentioned in EP 0,183,669 is the difficulty involved in removing the articulation element in the femur once it has been positioned in highly a stable manner. A precondition for removing the element is, therefore, that a hole be bored through the prosthesis joint surface.
Another important disadvantage of the femur prosthesis in the previously known prosthesis version is the difficulty involved in matching the bone bed to the covering part of the contact surface of the articulation element against the polyethylene component. In cross-section the articulation element has a mushroom shape, and it is assumed that it will be possible to adapt the underside of the mushroom cap to the bone seat, but how such a contact is to be achieved is not stated, and there are obvious difficulties in solving this cutting problem in surgical practice.